Diseases: Swine flu

Also known as Influenza

Swine flu is caused by influenza A virus, an orthomyxovirus with an RNA and protein core and surface spikes bearing haemagglutinating (H) and neuraminidase (N) antigens. Swine flu affects pigs of all ages.

In 2009, there was a worldwide outbreak in humans or what was initially called swine flu, also called Mexican H1N1 pandemic flu or later. This virus resembles but is different from swine flu and does not originate from pigs. Pigs can be affected but this strain is far less pathogenic than real swine flu. The danger from H1N1 infections in pigs or other livestock is based in possibly occurring modifications of the virus Resulting in Higher pathogenity.

Swine flu can be grown in embryonated eggs or on a number of porcine cell lines. Influenza viruses are sensitive to heat, drying, detergents and disinfectants. A number of different strains exist and are described by their H and N antigen combinations. The virus is very variable and is constantly changing its surface proteins.

Causes of Swine flu

Swine 'flu is caused by influenza A virus, an orthomyxovirus with an RNA and protein core and surface spikes bearing haemagglutinating (H) and neuraminidase (N) antigens. It can be grown in embryonated eggs or on a number of porcine cell lines.
 
Influenza viruses are sensitive to heat, drying, detergents and disinfectants. A number of different strains exist and are described by their H and N antigen combinations. The virus is very variable and is constantly changing its surface proteins.
 
Classical swine influenza is associated with influenza virus A HIN1 in the USA, and related strains of H1N1 virus occurring in Europe and many other parts of the world. The current human H1N1 strain can multiply in pigs and cause disease.
 
In many European countries, Japan and other South East Asian countries H3N2 also causes disease. Many H3N2 isolates from Europe are closely related antigenically to A/7/Port Chalmers/1/73, a human strain that appears to have persisted in pigs. The virus enters the respiratory tract and multiplies rapidly within cells of the bronchial lining until at 24 hours post-infection most cells are infected and mucoid pus is present in the bronchioles. Infection has largely disappeared by day 9. Infection strips the bronchial lining to stop secretions being cleared, giving rise to pneumonia and allows other respiratory pathogens to invade.
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Effects of Swine flu

  • The incubation period is 1-2 days and there is rapid, virtually 100% involvement of all susceptible animals.
  • Affected animals are apathetic, will not rise, have a reddened skin, inappetence and fever (to 41.8°C, 107°F).
  • Coughing occurs commonly and may be sufficiently severe for pigs to appear to vomit mucous.
  • Sneezing and difficulty in breathing are accompanied by reddened eyes and conjunctival discharge.
  • Loss of condition rapidly becomes apparent.
  • Recovery usually occurs suddenly at 5-7 days after the onset and very few pigs die (usually fewer than 1%).
  • Growing pigs may reach slaughter up to 14 days later than expected.
  • Piglets may develop disease at 2-5 days of age and adults kept in cold conditions may develop the clinical signs to a greater extent.
  • In some herds infection may be completely subclinical and only be detected by blood tests for antibody.
  • Outbreaks may end quickly or continue in finishing pigs with new cases for up to 7 months.
  • In a typical outbreak only a few animals develop acute signs, others are scarcely affected and the disease spreads slowly.
  • Influenza may be followed 3 days to 3 weeks later by abortions in sows in the second half of pregnancy.
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Diagnosis of Swine flu

Clinical signs of a rapidly-spreading respiratory disease affecting all pigs in a non-immune herd suggests that the disease is influenza ('flu). The dramatic illness with coughing, sneezing and fever and failure to rise or eat of affected sows is almost unmistakable.
 
The disease is less easily identifiable in herds which are partially immune as coughing, sneezing fever and inappetence may only occur in individuals as their maternal immunity wanes.
 
Diagnosis is confirmed by a rise in antibody in paired serum samples taken 3-4 weeks apart. Antibodies can be detected using the haemagglutination inhibition (HI) test and ELISA tests.
 
The virus involved in the outbreak gives best results in tests, so new variants may be hard to detect. Virus isolation from nasal or tonisllar swabs is possible if samples are taken within 2-5 days of the onset of clinical signs.
 
Sharply demarcated purple-red pneumonic lesions are present in the apical and cardiac lobes of the lung in affected pigs and may occur in the other lobes. Mucus and pus are present in the bronchi and the mucosae (linings) are congested. Virus may be detected microscopically using fluorescent antibody to N1N1 and H3N2, immunoperoxidase or by Polymerase Chain Reaction (RTPCR) for viral nucleic acid.
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Treatment & Control of Swine flu

  • There is no treatment for 'flu.
  • Secondary infection with bacteria can be treated by injection what an antimicrobial as food and water intake is erratic during acute disease.
  •  Ensure that affected animals can drink and provide supplementary feeding for piglets with affected mothers.
  •  Effects on reproductive performance can be minimised by purchasing semen to supplement boars, which may have had fever, and by serving sows when body condition has improved
  •  Spread of the disease may be reduced by preventing contact and by disinfection.
  • Control in infected herds is by vaccination. Two doses of an oil adjuvanted vaccine, 3 weeks apart, has been used in Europe to protect against both clinical disease and loss of production. It contains European H1N1 and two H3N2 viruses to protect against the most common strains and is updated with new variants as they appear.
  • Isolation of seronegative herds and purchase of seronegative stock can protect against introduction of disease; mainly by the introduction of infected pigs which can carry the virus for up to 12 weeks after infection.
  • Wind, birds and infected workers may all introduce the disease, so isolation cannot provide complete protection.
  • Total eradication from enzootically-infected herds must be by depopulation and restocking.
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